1. Nine male and six female healthy subjects were studied during supine bicycle exercise at workloads of 12 and 37 W; pedalling rates varied between 30 and 50 cycles/min at each workload. Measurements were made of oxygen consumption (VO2), carbon dioxide production (VCO2), minute ventilation (VE), tidal volume (VT), respiratory frequency (fR), inspiratory and expiratory time (TI, TE) and mean inspiratory flow (VT/TI) using a non-invasive canopy-computer-spirometer system. 2. At rest, males had greater values of VE, VT, TI, inspiratory duty cycle (TI/TTOT), VCO2 and VO2, and a lower fR, than females. 3. At the lower workload, VO2, VCO2, VE, VT and VT/TI increased linearly with increasing pedalling rate, whereas at the higher workload there was a decrease in VO2 and little or no change in ventilatory parameters from 30 to 50 cycles/min except for an increase in fR in females. 4. While performing supine exercise, there was an effect of pedalling rate on ventilatory and metabolic parameters at the low workload (12 W) which diminished at the higher workload (37 W). An increase in pedalling rate appears to enhance efficiency at these low workloads. 5. Differences between the sexes during exercise generally include: (a) a higher breathing frequency, (b) a greater mechanical efficiency, and (c) lower ventilatory equivalents of O2 and CO2 (VE/VO2 and VE/VCO2) during the higher workload in females than males.